Circuits for rapidly charging batteries are known. "Rapid charging" is commonly used to refer to the technique of charging a battery at a voltage and/or current which exceeds the nominal rating of the battery. As a result, rapid charging provides for the charging of a rechargeable battery in a shorter time as compared to a normal charging operation.
Unfortunately, there are several drawbacks associated with existing rapid charging circuits and the circuitry used to connect the rapid charging circuit to the battery. For example, typically it is desirable to recharge the battery while the battery remains in an electrical device being powered by the battery, such as a cordless telephone, power tool, personal pager, etc. One of the problems associated with existing rapid charging circuits is that the voltage and/or current that appears across the terminals of the battery while connected to the device is too high for the internal circuitry of the device to withstand. Such a high voltage and/or current often will result in damage to the device. As an example, a rapid charging circuit may deliver ten volts across the terminals of a five-cell nickel cadmium battery pack. This is compared to the nominal six-volts typically provided to and from the battery pack. As a result, the internal circuitry of the device can be damaged, particularly when the device is directly connected to the battery and does not include a voltage and/or current regulator.
Thus, it has been necessary in the past to remove the rechargeable battery from the electrical device and to place the battery in a stand-alone rapid charging circuit in order that the electrical device will not be subjected to high voltage and/or current during rapid charging. The removal of the battery from the electrical device is, of course, time consuming and can often result in the battery being misplaced, dropped, etc. upon removal.
Still another drawback associated with existing rapid charging circuits and the related circuitry used to connect the rapid charging circuit to the battery is that the rapid charging circuits and related circuitry do not provide for the safe operation of the device while the rapid charging is occurring. Typically, existing rapid charging circuits do not sufficiently isolate the battery from the internal circuitry of the device during rapid charging so as to permit simultaneous operation of the device using, for example, external power. Furthermore, existing rapid charging circuits and related circuitry do not provide for fast and convenient switching between power delivered to the device by an external power source and power delivered to the device from the battery.
In view of the aforementioned shortcomings of existing rapid charging circuits and the circuitry used to connect the rapid charging circuit to the battery, it is apparent that there is a strong need in the art for a switching circuit which permits the rapid charging of a rechargeable battery even while the batteries remain in a device, without damaging the internal circuitry of the device. More particularly, there is a strong need in the art for a switching circuit which permits the rapid charging of the battery simultaneously with the operation of the device, especially in the case where the device is nonregulated with respect to the power coupled to the internal circuitry. Moreover, there is a strong need for a switching circuit which sufficiently isolates the battery from the internal circuitry of the device during rapid charging to prevent damage to the device. In addition, there is a strong need in the art for a switching circuit which provides for fast switching between power delivered to the device from an external power supply and from the rechargeable battery. Furthermore, there is a strong need for a switching circuit which does not require that the battery be removed from the device prior to rapid charging.